PORTLAND, Ore. - A SWAT team searched a wooded area south of Portland on Thursday for a 13-year-old girl who is believed to have run away with a 40-year-old homeless man.

Samantha Mae Dodson was last seen Tuesday night at her Oregon City home, and her family reported her missing the next morning. It's believed she willingly left with Kelsey (Patrick) McCune, a friend of Samantha's father who spent Thanksgiving with the Dodson family, according to the Clackamas County Sheriff's Office.

The SWAT team went to Newell Creek Canyon because it's near Samantha's home and popular with homeless campers, said Sgt. Robert Wurpes of the sheriff's office. The FBI and other agencies were assisting in the investigation, which is being conducted with unusual urgency for a case involving a suspected runaway.

"This one's a little bit unique, and that's part of the investigation that I can't talk about right now," Wurpes said.

Extra personnel have also been deployed because the pair is likely sleeping outside during a cold snap with overnight lows below 20 degrees.

Samantha's older sister, Sarah Dodson, said Thursday that the middle school student is a talented illustrator who wants to someday be a scientist or marine biologist.

"She's just a kid, and she can't be out there on her own," she said.

The sheriff's office has declined to say what evidence led investigators to conclude the girl is with McCune.

Samantha has no history of suicide attempts or running away from home, the sheriff's office said. McCune does not appear to have a criminal history and is not considered a danger to the public.

"When I think about my own daughter missing from home with a 40-year-old man, that alone is enough to make some real concerns about her welfare," Wurpes said.

Samantha is 5 feet, 4 inches tall and weighs 125 pounds, with brown eyes and dyed red hair. McCune is 6 feet tall and 170 pounds. He has a glass eye and salt-and-pepper hair.

Though he has no known address, McCune makes money doing freelance computer work. Police said he may be carrying large duffel bags or backpacks with camping-type equipment.

Wurpes said there's no indication McCune has a car, and the pair may be using public transit to get around. Clear Channel Communications Inc. plans to put up three "Missing & Endangered" billboards in the Portland area.

Samantha's parents, John and Stephanie, released a statement Thursday afternoon in which they asked their daughter - one of five children - to return or at least let them know she is OK.

"Sami, your mom and dad are worried about you. Your brothers and sisters are worried about you. Your friends are worried about you. It's cold out there, and you just can't be out in that cold."